This invention relates to electrostatographic reproducing apparatus and more particularly to a method and apparatus for detacking electrostatically a toner receiving substrate tacked to an imaging surface.
In the electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder, referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as a copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface, the photoconductive insulating surface is cleaned of residual toner to prepare it for the next imaging cycle.
In such apparatus' it is common to electrostatically transfer the toner image from the imaging surface to the support surface, such as paper. This is typically done by charging the copy sheet from its rear face to the same polarity and substantially the same potential as the potential in the electrostatic latent image. In this process, electrostatic forces are created which cause attraction between the photoconductive layer and the copy sheet so that the copy sheet tends to adhere to the photoconductive layer. Previously, several mechanical means have been proposed for stripping or separating the copy sheet from the photoconductive layer. Typical of these devices are the mechanical fingers, gripper bars, and other devices which physically grab at least the lead edge of the copy sheet. The difficulty associated with these is that in the process of gripping the lead edge of the copy sheet it is possible for image deletions to occur. Other mechanical devices used in the past have involved the use of puffers which try to blow a thin stream of air between the imaging surface and the tacked copy sheet. Like the fingers and grippers, the air stream from the puffers tends to displace the unfixed toner from the copy paper when it is blown between the copy paper and the imaging surface resulting in toner being displaced throughout the machine thereby contaminating sensitive parts. Furthermore, such devices are relatively bulky and expensive in construction.
Another device to have been attempted is the use of a conductive tinsel type device in contact with the paper which relies on the placing of a charge on the back of the paper of a polarity opposite the polarity of charge placed on the paper during transfer. The difficulty with this is that tinsel has a tendency to totally discharge the copy paper where contact occurs resulting in no charge holding the toner in image configuration on the paper. Furthermore, the charge on the photoconductor may be sufficient to keep the majority of the toner material on the photoconductor rather than have it transferred to paper. Thus, in areas where the tinsel has a tendency to totally discharge the paper, the toner will go to the drum resulting in image deletions.
A further device which has been successfully used is the use of a detack corotron which generates a corona discharge of alternating polarity thereby neutralizing the charge on the copy sheet. While this is capable of producing satisfactory detack, AC detack corotrons require a high voltage power supply, are normally very expensive, and the power supplies occupy a relatively large volume. Accordingly, they are particularly unsuitable for use in relatively small, compact, inexpensive copying machines. Furthermore, the wires sometimes break and must be replaced and the alternating current corona may produce excessive amounts of ozone.